Corrosion inhibiting composition containing an aminoalkyl-phosphonic acid and an inorganic nitrite

ABSTRACT

The present invention relates to a synergistic composition for inhibiting the corrosion of metal surfaces, particularly ferrous metals, in contact with aqueous systems comprising a combination of an inorganic nitrite and an aminoalkyl-phosphonic acid.

Skates Pa 19211 1191 2 .1111 B I. I

Harris/e: 0!.

[s41 coaaosioummamm comvosmo 5 v jg/ med (IONTAINING AN UNTED-STATES NT'S AWNOALKYL'PHOSiHOMC AND 2.351.465 6/1944 .wa1i1er....;;1-..l.. "3252/3117 AN I R ANIC NITRI 3.4253254 2/1969 J 12616111611619. 2527389 R lnve ntors: Arthur Han-is, Poyntor1; John Burrows, Conglemm James Roger Hargreaves, Sale; 1" of England Assighe: Ciba-Geigy Coma- 116011, Ardsley Filed: June 22, 1972 Appl. No.1 265,369 I Y Published under the Trial Voluntary Protest Program on January 28, 1975 as document 110.

' 1 6161 11 11 116111611 PIEOS'iW Data June 26, 1971 111111611 Kingdom 30064/71 3,433,577 3/1969 -Shik 3,483.133 12/1969 116161 61 al. 252731194 3,532,639 10/1970 Hatch-....'. 252/339 A 3,668,138 611972 Hoover 61 1.. 252 387 3,720,498 3/1973, RCdmOI 1. ..-252/3s9 A 3.723.347 3/1973; 1911 611611. 4 2421311916: 1

P/ima/y Ekahzir'zeL-Benjamin R. Padgett 7 Assistant ExamineF-lrwin Gluck At t0rney, Agent, or'EirmCharles W. Vanece k;

Nestor W. Shust l [571 Y mm/11:2"

The present invexition relates to a synergistic cor/117051 iion for inhibiting the corrosionbf 1111:1211 suffaqgs,par-

2 2 6 mm Fi ures;

Patent f Dc. 9 1975 Sheet-2 of ferrous metals, in contactwith aqueous-systems;

In the past, water-soluble chromates and dichromates have been widely used for inhibiting the corrosionof metals incontact with aqueous systems. However; chromate and dichromate ions are toxic and pollute any stream of river into which water containing them is discharged and are therefore ecologically unaccept able. Inorganic nitn'tes have been used as corrosion inhibitors, but it is usuallynecessary to use them at high dosage levels. often as much as 650partsper million. Amino phosphonic acids have also been proposed as corrosion inhibitors, usually in conjunction with zinc or chromate ions or tannins since when used alone some amino phosphonic acids have the disadvantage that, being sequestering agents, they promotedissolution of themetal with formation of complexed ions. I i

A major advantage of inorganic 'nitrites their cheapness, but this is offset by the fact that itis necessary to use them at high dosage levels. Clearly it would be most advantageous if a corrosion inhibitor combination could be devised,' based on nitrite for cheapness,

\vhich had at least the same efficiency at much lower dose rates, that is if a suitable compromise could be reached between efficiency, doserate and cost of use at that dose rate.

Surprisingly we have knowledge ofthe properties of each. that is to say these compounds can form synergistic combinations. The mechanism of synergism is not'simple; in chemical synergism, cases are wellknown of two or more materials which demonstrate significant synergy when combined in'certain ranges of proportions, but whose combined found that the combination of f an inorganic nitrite and a member of certain classes of aminoalkylphosphonic acids can result. in a corrosion inhibitor whichis better than could-be foreseen froma trogen atonnfl is hydrogen or an'alkylene phosphouic earbbnammaAiissmtui-ated acid containing up to 4. g V divalent hydrocarbyl radical containing up to 6 carbon atoms, m is Oor an integer from i to n i's tl'or 1-, when m is 0, R may be a hydroxy groupingor R and v R together with the. nitrogen atom membered' ring; when m;- is; l," the grouping RN-A -NR may form a hetercyclicring and when m is greater than 1; the constituent)! groupings may.

both: same or diferent; andz the' water-soluble salts i may be the alkali metal and ammonium salts orthe salts of organic basesQfor example, the mono, and tri-ethanolamines, 'alk yl and arylamines, and guanidi'ne.

In this specification, we define synergisrtiintemts of the graphical interpretation-of theiesults derived from,

a simple test for corrosion inhibition properties of ma terials. in this test, the effectiveness of anadditive'as a corrosion inhibitor in-aqueous solution is measured in termsof the loss ofjmetal from a test'specimenfimmcrscd in the aqueous solution under standard condi tions. 'A result is obtained to show theefficiency of inorganic nitrite by itself as a lcorrosion inhibitor, and a result is obtained to sl'tow the efficiency of the aminoah kyl phosphonic acid under test by itself as a corrosion inhibitor these are plottedon a'graphas'thefi'gures for 100% inorganic nitrite and 100% aminoalltyi'jp' lto's phonic acid. Combinations such as 8U% "ttitritc[2Q% aminoallryl phosphorus acid. 60%nitsitel40ib aminoalv kyl phosphonicfacid, 40% nitrite/% mumm il s;

phonic acid and 20% nitrite/% arninoaikyl phosphonic acid are thentested until a clear graph can be effect is only additive (or even worse than additive) at other proportions, and this is thesituation in the pres ent case. All combinations we shall describe show synergism as we shall define it when combined in some proportions; in other proportions. the combined effectmay merely be additive, or can be antagonistic.

Accordingto this invention. there is provided a ergistic combination (as hereinafter defined l cornprising a combination of an inorganic nitrite and aicompound having the formula:

M on

each represents hydrogen or a hydrocarbyl or hydroxysubstituted hydrocarbyl radical containing up to 8 carbon atoms. R and R. may be the same or different and up to 3 carbon atoms, a phosphonic acid. an allrylene plwsphonic acid grouping containing up to 4 carbon atoms or an aralkyiene phosphonir: eeid'sush thtitnot wherein R and R be the some or different and plotted to show theefiectiveness as afcor rosioninhibn tor of all combinations oi'the vi titriteanri aminoalkyl phosphonicacid under testfThisgraphis uma-ane at thesame time linear interpolation between the values forlfifllia inorganic nitrite and lflfi arninoalkyl phosphonicacid is drawn. We detineacombination'oi inorganic nitrite nndaininoalkyl phosphonic'acid as a" "synergistic combination iatthe valuefoi'wrrosive-weight loss observed'in practice by @mbfithiitesttiMq! or less than%.% of the value ofjj'the corrosive weight I loss for that combination on the linear interpolation be. 7 a tween the values for inorganic nitrite-andllm aminoalkyl plwsphonic acid.

The test method we use will bepreferred to as;

Brittle Test and is describedindetail r Aerated-Solution asfollows:

The apparatus more described with reig ite; is M3. 1 accompanying drawings. FlGI i is}: longitudinal t" tion of the bottle'used for the-testto one a length of giants tubing cementedto tlic-ihside'oi-thahjiiitte using Araldite" resin A'Ylm in conjunetionvrith' hardener moss. About 2 grams of thentixture ior hotti'e is usually required FIG 2 is a transverse cross-section of the home usedfor the test.

FIG. If? a longitudinal section of-theinittie intrsc during the test showing the outse -attentional contents.

Fit'i. d is rt contracted longitudinal aecsiiihoth water" 3. p I I H6. is a ems-section of a water distributor with a T-piece. I FIG. 6 is a perspective view of the whole apparatus.

, Referring to H68. 1 and 2. the bottle. 10. having a volume of approximately 130 millilitres has attached to the inside 'wall by means of cement. ll, a glass chimney. 12, which is 2 inches in length and positioned so that thereisa inch gap between the bottom of the tube and the bottom of the bottle.

In FIG. 3 the bottle. 10, is shown with a mild steel test coupon. 24, suspended from the neck of the bottle by a nylon thread, 25, which is held in position round the neck by a rubber band; 26. A piece of polythene tubing. 19, dips below the surface of the water. 27, in the bottle. 10, and another piece of polythene tubing, 28, having an internal diameter of 0.030 inches fitted with a jet, 29, passes into the-glass chimney. 12, and tenninates near-the bottom oftheglass chimney.

Referring to FIGS. 4 and 5, a distributor, 13, made from copper tubing 17 inches'lengthand having an outside diameter of )5 inch. is sealed at one end, 14. The other end. 15, is threaded to take a nozzle 'lo'and polythene tubing, of /4 inch internal diameter is fitted over this nozzle. Into the copper tube are welded 30 pieces of copper tubing, 17. each of which are 2 inches long. have an internal diameter ll l6 inch and are spaced 6 inch apart to form effectively 30 T-pieces. 1 i

In FlG. 6. the bottles. .10, are placed in a thermostatted water-bath. l8; Pushed over the ends of the '1'- pieces, 17, are lengths of polythene tubing. 19. approximately 12 to l5 inches in length and. 15 inch internal di ameter. A -litre water reservoir. 20., is connected to each of the bottles via the distributor. l3, and polythene tubing, 19, dipping below the surface of the liquid in eachof the bottles, ith'ihe reservoir is tightly s'toppered with a long glass tube. 21. passing through a rubber bung. 22, to the bottom of the reservoir. A small hole, 23, is made in the wall of the tube near the bot- In the test itself a sample of the mixture to be tested is added to a miiiilitresample of a synthetic corrosive water having the following composition Z0 grants CaSO JHzO 15 grams MgStLJPhO 4 .6 grams NaHCO;

7.7-grarns CaCl bhia 45 gallons of distilled water. The concentration of the mixture to be tested is i parts of mixture per million parts of synthetic corrosive water.

in hydrochloric acid. dried and weighed. is suspended below surface'oftire solution. The solution is then if; stored?! 409C in the-thermostatted water bath.

{ During the storage period. the bottle is utinuouely aerated by passing air. (SOtljmillilitres/rninute) through the tube, 28, screened fromjthe metal surface byfthe gins chimney, 12. The aeration ofthe bottle is c0ntrolled by a rotarneter calibrated-in litres per minute theme tubing. 28. with the jet 29. Water losses caused by evaporation are replenished' vvithfdeicnised water dispensed from a constant headdei/iee. this being the- A mild steel test coupon (5-3 2.5 centimetres which scrubbed with pumice dipped for one minute .ZO-litre water reservoir which in eonnect edto the db: tributor. 23 by polythene tubing andttwn to the bottie containing the sample podtio n of the giant tube -2i, in the reservoir is 5 justed so thatthe level of the hole, 33, eoincidee with 'therequired water level in the bottle undertest, ix. Atter 48 hours the mild ateelcoupo'n is -removed,

(m.d .d.). Each test is carried out on duplicate specimens. Y

be selected from the general formula I. Class I having the formula: v

derived from the general formula lwh'ere n Oandit'. 2S

hydroxyl substituted hydroca'rbyl. R and R m represent carboxyalkyl. or R and R together with the nitrogen atom may form a 6-membered'ring'. and the totai' number of carbon atomsfin-the groups R and R 1gether does not exceed lOI ilhe'hydrocarbyl or hydroxy substituted hydrocarbyi group may be: v a1 ailsyl. forexample methyl. ethyl. mpmpyl, 'isoprof cycloalkyl. for example cyclohexyi g. hydrcmyalieyi. for example:

HO tu cu Thecarboxyalltylgroup methyl or fi-carboxymethyl groups.

by-rnoie polythene mm. m

scrubbed with pumice. dipped in hydrochloric acid inhibited with 1% by weight of hexamine for one minute;

' The corrosion rateis recorded as the weight loss in I The following particular of compoundsimay R, Rahd R may represent hydrogen, hydrocarbylor e. aliienyl. for enampleallyl, or-methallyl. crotoriyl I p The six-me'mhered ring may be for exampiez- Q derived from in: formula I when: "F0, R- and R" are both llydrogenf'li is a melhyleng phmplmnic acid gl'ouping and R is a hydrocarbylfor hydroxy substb acid grouping. Class V havfing lh'e'foxmulaz dgzivedfrdm thgfmmnlalwherem= n= LRishydrogen R may be hydrogen or a hydroxy phenyl gxoupmg. R and R are th: same and arehydmgcn or mezhylcne phosphonic acidgmupinpand A is asaluratad di valent bydmcarbyl, which may helium: or branched.

top've a yl grooms; and Win a mzthylene phoapbonic acid the lcrmuib gouping. Class Vl having derived from formula I where m isa positive in'aegr,

510m 2 to 60, n LAB; an emyim grouping R, R

l and I l my web be hydrogen or a methylem: p-

and dis; m: 856ml? muping be: identical/ v :l wmwunds f l are Mlazzeiokg;

m lj wmyl plsmplwsiic acid. maul-lino will czwly, Has. no 125m the a'ctompanyirg dm acidmouping providzd that m least 50% of than! a we methylene phoaplmnic acid @mupe,

kmmemyzne p =honic acid); n lwsylamim plmnic acidj p-tolu'idind dimcthyleneiphqqpho An example of a of v J mum-ma and-c rb n nmh' p I aminp di(mmylenejphosphonic.acidymul byglmxy amino di(methylene phosphouic acid). An exannple of; I

a compound of Class gv'gs hydmzine t'4:tta(l'w etliyler e phosphonic acid).

- An example of a compojlmd of V is e thyleh dif amine tetra(methylene phosphonic acid).

derived from formula lwherin m i O, R is hydrogen 9 An x m piclmsivlf a 99 or a mqthylche phosphonic acid grouping and R is a pmmd P T mm hydroxy, -CH, CO )H or a -CI-LClhCOOH gouping. Clas IV having the forrmxla:

Ho\ 0 H 0 PCH N CH N PCH cu 1 v l no /u N on cu m a derived from fonnula] whf ein R"a.nd R are hydro- VIII .gen. R is hydrogen or a methylene phosphonic acid m g 5% 2 l grouping, m l. n 0. R a hydrogen or a methylime 1 1 r 1 phosphonic acid gouph'ng and R is a methylene phosv I 4v .film wl I The compositions of the premzm invention colzp ain I from 10% to 99% by weight 'of an inorganic nitrite and "from (xi-HQ by weight of an mninoalkylphosphofiig and having the formula 1. Preferred synerg'stic ob'mbinatinm are 00:11am ccmwining, a.cjampound of Class I; 2!, V opVMnhombinmim with from l m.

5 v by weight 9f wifium nitrite; compositns cozimm ing a compound oi Clam ll! in combinatiomwim 101% w 90% by weight of sodium nilrifzjaigd clans oomaining a compound if Clam W tien with from 10% to 80%by weight of waiiiam nitzlm. In prder to syneygsm nbtainadlmore --we-re platted to'showfiw 'efiecisncy as gtmfmiqn inhi lyb tom of'mixmms of sodium nitrite and a of each ofthe clam 1 m6: F168. 7. no 12 valiafilm of #1 dqs'miion 'l atio against sa drum mm m the magma. The eoqmwn ratio M fin sd as the wmwme in the o f fln: mixwe divided by the on rate inthc pm of l sodium rd'tsita, The valuafor 'lm' nb we. 1 mm Elm syneygiam obtained all milla e acid). a wmpmami af 2m; y l v MG. 3 53mm am synergiam alumina! fc ra mmw 5 cm'maining hyclmzw tmralmmylma' .lplsmplmmiz" 3 1 honic nenmo fimcwv pound of Classy; and 1-16. 12 shows the synergism of phonic acid.

, 7 gism for methylamino an methylene phoaphoniczacid); aeompound of Class III;

ll ahowe thesynetgism obtained for ethylene diamine tetratrriethylenephowhonic acid). a com- P mp a id). ethylene diamine -tetra(methylene phosphonic acid), mcthylamino' di( methylene phosphonic acid) and a-anilino ortho hydroxy benzyl phos- Particuiarly preferred synergistic combinations of the present invention are those where the value of corrosion observed in practice by means of this test is 50% or less than 50% of the value of the corrosion for that cornbinationon the linear interpolation between the i values for 100% inorganic nitrite and "1 aminoalltylphosph'onic acid. As can be seen from the graphs. mixtures which give such a corrosionival'ue are:

LMixtures of methy'laminodii methylene phosphonic acid and sodium nitrite containing from 30% to 85% sodium nitrite.

2. Mixtures of hydrazine tetra(methylene-phosphonic acid) and sodium nitrite containing from 22% to 30% ot sodium nitrite.

3; Mixtures of a-anilino ortho hydroity benzyl phosphonic acid and sodium nitrite containing from 30% to 85% of eodium'nih'ite. v y

4. Mixtures .of ethylene diaimine tetratmethylene phosphonic acid) and sodium nitrite containing from 25% to 85%;sodium nitrite.

Mixtures of compound of Class VI where the av-, erage value of m is 41.5 and sodium nitrite containin'g from to-SOk'sodium nitrite.

The nitrite that is used may be an alkali metal nitrite, for exampie. potassium nitrite, but is pret'erabiy sodium nitrite.

The present invention also provides an aqueous systern containing a synergistic comhination as i'rereinlve fore defined.

Furthermore there is provided a method of forming or replenishing an inhibited aqueous solution which comprises adding to the system inorganic nitrite or a compound of formula i or both in such quantities that ma aria at their additiongia o e a n hei'einbefore. defined. Conveniently, the corroeionin h an amount I".

or-chromi nnions'or aminee. y I furtherillustrnte the present; invention. Parts and percentagea are errpreeeed "by hib i z mixture is we n? 10,000 per fl'lilliOn-OfjitQ. aqueous solution; l-low-, ever amounts greater silicate to controlattack onaluminum-or' a specific in.- hibitor to control attack on. (or induced bylcoppei There may also be added other corrosion inhibitors to ferrous metals known per 3e,

The fol lowing'lfixamples weight. EXAMPLES 1 we in of the followingfixamples, four mixturea" were made up from wtiiurn nitrite 'anda compound 0 formula I as illustrated in Table I. each mixture c on 'taining 20%. 40%, and'80% by yveight'otthe com pound of formula 1 respectively. Samples of Jot theee mixtureswere added-to 'lOOmillilitie sample'soi t y the agressive as hereinbeiore defined water so that four. v

solutiori s were preparedeach containingfltfl) parts per millionof the mixture. Theaerated bottle test vas car ried out .on these soiutions., i

In order to demonstrate the synergistic efiectzot" the mixtures of the present taining sodium nitrite and the resp'ectivecomponnd of formula were tested separately in a similar manner at concentrations of parts per ra ults are shown in Table :1.

Theresulta obtained clearly ehow the 'Synergiitm -ot" mixtures of the present invention.

EXAMPLES 9 to 20..

The results are shown in T'abic 2.

mus

Corrosion Euro cl mizturea cantataarying ercentage: o. g nymznd 0F Y'Jf lul: I FXI ViQ- 1 Compound of Permit 1 190 am am 20% m;

t crgmcu m n n 3 (L59 OJ, y

2 i !l(CR P0 ri 1 H; 0.6? (3.51 0 4 6.!) 1.0; 5 tri tcfl i mca ro ug 1.79 mi 0.1% m (1.39, .7 4 CMZ-K;NC2MCH2P0)H2)2 0.37 0. 9 0.05 0.21 the; xiii?- s r'. l! W{CH F') H ztzs 0,91 0.37 0.01 mm i a *u mm in t') r s n w n sn on .v .7 r

1100 parts per'million 5 may w -mo pt when added. shock dosesn" v lfdesired there mnybeaddedto the mixturmapecific corrosion inhibitors for non-ferrous metals suchae' a? e.g., zinc,

inventionwhen comparedwith; 1 theindividuatconstiments'of the mixture; samples'cow' miilionl Thecorro eion 1 ratio was calculated as hereinbet'o're deiinedf'andgthe-f Somefurt h er laboratory lsts wer'e carried out using a Laboratory Heat Exchanger Rig a form bf labprzito'ry tsting intendedto simuiaiepractical use conditions more closely tha'n docs the Aerated Solution Bottle Test'ju'st described. in this rig, corrosive wateris aermctal tbupuris and the heat cxchim'gfl tube; are exammed, and their slate assessed.

In detail, the= rig consists-"0f a-cl0scd W316i circuit, mag; up of lhc'fullnwirig it rris in brnlcr; 2 litre fcsbfvoir 1 Hire rc sc rvir flow mgte'r cfiupon chamber hem ex chang'cr wuling'cundemcr cumprcssd iiir immduccd lhmugh a simurvd aiiw'c u litre reservoir. From this ruse rvoii' i1 i pumptpd t hrbufgh the flow melrfto thegiass'coumm 'cham bi in which I are a number of recluigukir-melz l (ibupons each lfiby I 51) Cms. mount-ed 0i) aipcrspii jig Thc wager .th'eri "flows through the heat" area} and circulated overz'a number ,0!" metal coupons. v i -and -is heqted ,byibeing passed through 31 healed steel" I heat exchanger tub e. Aftcr'a' suitabiqtelsi. period, he

xcharxger which a m-mch mlcmal dia .Cobling condenser to tiiii 21am rsler\' oirf zibkmi 5 litrgs per minute. and i {hen pumpud in tha: l'l

metal "acid i frheresults obtainedare illustrated Tehle iiuand 1- milrist eet Cone HCldiluted 1-:1 i

mm mm a mm s show quite clearly that SyJElgISVClC mixtures the p re copper 1 3 9 7 2 v ent'invention are atieast as goodasjifn'otfbetter than; v hm! v 'aluminium 5%,nhmpho i u lz corrosion. 1nh1b1t1ng-m1xtures conta ning; anus-,anrl/or Y I chrornic acid. at [5C. 7 I, v I I r 5 I chro'mate ions.

CORROSION RAT OF METAL coupoNs 1 ing. per sq. decimetrc per day Mild r 1 Run Steel- Copper Brass Aluminium V ition of HeatExchangeTube "1 e9 42' 15.6 sjs- SEVERELVYCORRODED i 1 2 42.5 1 s- I WHOLE SURFACE ENCRUSTED wrm HE V I CORROSION, I I, PRODUCTS 1 I 3 4.4 0.3 0.5 1 2.6 SMALL PATCHES OFCORROSKON. REST OF- runs I I p 1- SURFACE COVERED' BY A PROTECTIVE GREEN FILM I 4 1.8 I [.6. 1.0 i 1.54 do. -s 2.6 1.5 1.1 1.1 SURFACE PROTECTED APART FROM some I sMALLMrs-J 6 19.4 0.8 2.4 (1 7 CORROSION UNDER SCALE DEPOSIT After such immersion, the'coupons are rinsed with we clairn: I I w water, dried and weighed; they are then mounted on a I: l. A synergistic cornbination consisting .essentlally of perspex jig. care being takento ensure that none of the a combination offrom ll)% to 99% by weight of analcoupons-touch each other, and that they are insulated kalijmetal nitrite and from 90% to l by weight of a from the bolt holding the jig together. The heat ex- I compound having the-"formula changer tube is cleaned with pumice. dipped. in cone. i A v I hydrochloric acid diluted lz'l with water. and then 1 v 'rinsed in water and dried. 1 1.

The rig is assembled. and cleaned-thoroughly by cirv 1 I 4 f B culating conc. hydrochloric acid diluted lzl with .water,' x I N (Al- N P- then flushing with tap water for about half an hour 1 l 3 J (about gallons in all) and draining. The necessary R m R R OH .quantity of additives to produce the. desired concentrations is put into one of the reservoirs andthe rig is filled with 4.5 litres of a standardised corrosive test water, which is characterised as follows: m 40 v R and R may be the same or d fferent and each rep 522 x2212?! I 2 as w m of Cato resents hydrogen or a hyrocarbyl or'hydroxysub Total Hardness so istituted hydrocarbyl radical containing up to 8 car- Chloride l0 p'.p.m h atoms Conductance 2S0 I R and R5 may bethe same or different and each repv I The pump is primed and started. and the heater resents hydrogen one hydrocarbyl or hydroxy subswitchedon. stitdted hydrocarbyl'radical containing upto 8 can The-concentration of inhibitor and the water level in bon ato ms, a cartipxy-alkylor aik oxy radicai con the ng are checked daily and any losses made good. taining up to 3 carbon atoms, a phosphonic acid; I A a ndar Peflod of 0 y the eat xalkylene phosphonic acid groupingcontainin'g changer tube-isremoved. sectioned and examined; the to 4 carbon atoins' or an aralkylene phosphoni'c 1 test coupons are removed, cleaned as before except acid such that not'rnore than two phosphoni'c a'ci'd I that-the acid is inv each caseinhibited with 1% hexgroupings separated by less than twocarhon'atorns, m n insed, dried and retveigherlv The results hmay be attached to any nitrogen amine .'served enable an assessment to be 'made of the anti R is hydrogen oranalltylene phosphonic acid on scaling and anti-corrosive action of the inhibitor under mining up to 4 carb0n atoms. 3 f I test. A ma satu rated divaient hydrocarbyl'radica! cc t'ttaim'.- The following additives were testedv all parts and pering 'up to 6 carbon atoms. I centages beingby weight: I m is 0 or on integer-from tofith Run Addllwcs and Cwzuentruunn 5'" l Horn: 3 2 ppm Sodmm nitrite 2U; mcthylumuu .hnwthylcnc tllplimtplmmc I 3 20 1 ppm and p 7. 5m; 'n'itil l"! iilHlt 2W3 nit-thyLmum': (lunrlliylcnc thphmpllwiic 4 lnflml have! :1! MIN ppm LlCltl illl'i :tmlturn mint:

l lnuczl by mzumtrnnmr us I'm' Run fiit ppm iym rg'iwtl 71m It limm. ll" I'JIK I 44; t'rn; 1 Hill pm fun: lnlyplmtplhilv, l fi-I'nit.

' 1 it co m in a 0. a m be a hydmxy grouping or R and R together with the nitrogen atom may form a six mcmbere'dwring; when m is l, the grouping RN-A-NR may form a hcterocyclic ring. and whenm is greater than l. thc constituent phosphonicacitl n+hexylemino ditmethylenc phonic acid 'p toluidino' -di( methylene R groupings maybe the same or different; and the water-solublesalts thereof. i 2. A synergisticfcombination as claimed in claim 1 in which the water-soluble salt of the compound of forf' 'rnula l is the alkali metal or ammonium salt.

3. A synergistic combination as-claimed in claim 1 in which thealkali metal nitrite is sodium-nitrite.

4-. A synergistic combination as claimed in claim .1,

comprising a mixture containing from 10% to 99% of sodium nitritefand from 90% to 1% of a compound having theforr'nula I, wherein R and R may be the same or different and each represents hydrogen or alkyl, aI-J kenyl or cycloalkyl containing up to 8'carbon'atoms, R I

; and R maybe the same or different and each representshydrog en or alkyl. alkenyl or cycloalkyl radical.

such that not more than two phoaphonic acid groupings containing up to' 8 carbon atoms, an alkylene phos phonic acidigrouping containing up to 4 carbon atoms f-separated byless than two carbon atoms may be atv tached to any nitrogen atom, R is hydrogen-or an al-: kyiene phosphonic acid' containing up to .4 carbon.

-atoms, A is an. alkylene radical containing up to 6 carbon atoms, m is Q or an integer from 1 to 60, n is O or 'l, and when'm is greater thanl, the constituent R? groupingsimay he the same or different, and the water'- soluble. sals thereof.

5. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula where R R. R and R may represent hydrogen, hy-

' dr'ocarbyl or hyclroxy substituted hydrocarbyl, R and R may represent carboityalltyl. or R and R together with the nitrogen atom may form a 6-membere i ring,{ and the total number of carbon atoms in'the groups R and R together does not exceed i0. 6. A synergistic combination as claimed in claim 5 in I which the compound of formula I! is a(2-acctoxy anilino) benzyl phosphonic acid. ,a-anilino orthohydroxy benzyl phoziphonic acid. pipen'clino methylenephosphonic acid or amino methane phosphonic acid.

7. A izynergistic combination as claimed in claim'l in which the compound of formula l hasthe formula III hereR' and it are both hydrogen, it n a methylene P v hyrlrozysubstituted hydrocarbyl radical.

acidgr'nuping and R ia-a hydrocarbyl'o acid) or hydroxy' jth'yl 'lmiito di(me th rel e imminacid), n-pentylamincditmethylene phosphomcacitl);

n-propylaniino 'diutiethylene "phosphonic acid). '3 lylamino' di( methylene 'phosphonic acid). or cyclohex' ylamlno di(methylene. phosphcnic acidh" 9-. A synergistic combine n as claimed 0. on n 10. A synergisticcombinatio'n as claimed inclairn .9 in which the compound offormula W is hydrbxy'amino methylene phosphonic acid, carboxymethylaniinodi(-' methylene phosphonic acid) or hydroxy di( methylenepho'sphonic acid).

1!. A synergistic combination as inwhich the compound of formula lhas'th' formula.

whe'rein'each of the groupings isi-hy tltogcnor' a methylene phosplionic acid grouping.

I 12. A 'synerg'wticicombination as 'claimedin' I in which the compound of fonnula'yis hydrazine te l ra(met.hylene phosphonic acid); v

BJA synergistic combinatkm as claimed in claim 7 in V which me compound mmmuu ill a man mam (in methylene phmplnmic acid l, ethyl amino dit methylene m-pheny lene or pPhcnylene grouping.

' i3. A syncrgistic combination as claimeti in claim 1 in which the compound of formula-l has the formula no a f a 0 "0H l -r I ll/ P-c'-N -A--n--c P v1 '2 5 2- 0H m n .3 R a where R is hydrogen or a hydroxy phenyl grouping, R and i are the same and are hydrogen or methylene phosphonic acidgmupinm and A, .isaklinear or,

branched saturated divalent hydrocarbyl. containing from 1- m6 carbon atoms; or the grouping RN -A-- NR" may form a ring.

14. A sync: fccomhi'nat'icn-as claimed in ciaim 53 wherein A, in the oor'npound of formula VL'is a methylene, ethylene, n-propylene. isopropyle'ne, n biit'ylcne.

15. A' synergistic combination as claimed amine tetraimethylene phecphcnic e'cidl. g

to. A synergisiicecmbinatinn as; claimed in cieimt wherein the compound of 'iormula W in etiiy lene diiin which the compound of formelai hat the'tonnela where m is a poeitiye intcgcrfr'oin 2 to tiikwr it anti it" may cesrh he hycln'vgcn 0% a n wthyiene plmanhimic which the compo lung of formula I htiathofdrihula wherein R in hydrogen or a methylene phoaphonicjacid claimeil; 1

3,925.25 acid grouping. ptbvided that vat least 50% of these 20. A synergistic combination as cltiilneti inclaimif mum are methylenc phosphonic acid groups. and that consistingeseenttally of n minture flmj i ut i 3 not all the?! grouping need by identical. ottho hydmxy benzyl plicfiphonic acid and .17; A synergistic combination as'claimed in claim 16 i 85% of sodium-nitrite. I I, V in which tlie' compotind of formula Y" is: 1 5 ZLA Synergistic combination as claimcd in claitn l a H2O3PCH2 v ori -H Mango 'vm citPo n d 5 2 l-l.5(ave ;-age) 18. A synergistic combination as claimed in claim 1 consisting esecn'tiallyof a mixtui'c containing ethylene T- consisting essentially oi" ii mixture containing 'mediamine tetra-,(methylene phosphonic ncid) and ftom;

to 85% by weightof sodium nin'ite'. 211A synergistic combinattonas claimed thylamino di(methylene phosphonic acid) and from 30 to 85% sodium niti ite.

19. A synergistic combination as claimed in claim 1 consisting essentially of a mixture containing a comt i 1 consisting essentially-oi a mixture containing hydrazine pound "of formula VIII and from 20% to 80% s otiiuinlt -tetra(methylene phosphonic acid) and from 229610 mm.

. by weight of sotlit m' nitrite'.

a :a' e a a" ssi 

1. A SYNERGISTIC COMBINATION CONSISTING ESSENTIALLY OF A COMBINATION OF FROM 10% TO 99% BY WEIGHT OF AN ALKALI METAL NITRITE AND FROM 90% TO 1% BY WEIGHT OF A COMPOUND HAVING THE FORMULA
 2. A synergistic combination as claimed in claim 1 in which the water-soluble salt of the compound of formula I is the alkali metal or ammonium salt.
 3. A synergistic combination as claimed in claim 1 in which the alkali metal nitrite is sodium nitrite.
 4. A synergistic combination as claimed in claim 1, comprising a mixture containing from 10% to 99% of sodium nitrite and from 90% to 1% of a compound having the formula I, wherein R1 and R2 may be the same or different and each represents hydrogen or alkyl, alkenyl or cycloalkyl containing up to 8 carbon atoms, R3 and R4 may be the same or different and each represents hydrogen or alkyl, alkenyl or cycloalkyl radical containing up to 8 carbon atoms, an alkylene phosphonic acid grouping containing up to 4 carbon atoms such that not more than two phosphonic acid groupings separated by less than two carbon atoms may be attached to any nitrogen atom, R5 is hydrogen or an alkylene phosphonic acid containing up to 4 carbon atoms, A is an alkylene radical containing up to 6 carbon atoms, m is 0 or an integer from 1 to 60, n is 0 or 1, and when m is greater than 1, the constituent R5 groupings may be the same or different, and the water-soluble salts thereof.
 5. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 6. A synergistic combination as claimed in claim 5 in which the compound of formula II is Alpha (2-acetoxy anilino) benzyl phosphonic acid, Alpha -anilino ortho-hydroxy benzyl phosphonic acid, piperidino methylene phosphonic acid or amino methane phosphonic acid.
 7. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 8. A synergistic combination as claimed in claim 7 in which the compound of formula III is methyl amino di(methylene phosphonic acid), ethyl amino di(methylene phosphonic acid), n-hexylamino di(methylene phosphonic acid), p-toluidino di(methylene phosphonic acid) or hydroxy ethyl amino di(methylene phosphonic acid), n-pentylamino di(methylene phosphonic acid), n-propylamino di(methylene phosphonic acid), allylamino di(methylene phosphonic acid), or cyclohexylamino di(methylene phosphonic acid).
 9. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 10. A synergistic combination as claimed in claim 9 in which the compound of formula IV is hydroxy amino methylene phosphonic acid, carboxymethylamino di(methylene phosphonic acid) or hydroxy amino di(methylene phosphonic acid).
 11. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 12. A synergistic combination as claimed in claim 11 in which the compound of formula V is hydrazine tetra(methylene phosphonic acid).
 13. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 14. A synergistic combination as claimed in claim 13 wherein A, in the compound of formula VI, is a methylene, ethylene, n-propylene, isopropylene, n-butylene, m-phenylene or p-Phenylene grouping.
 15. A synergistic combination as claimed in claim 13 wherein the compound of formula VI is ethylene diamine tetra(methylene phosphonic acid).
 16. A synergistic combination as claimed in claim 1 in which the compound of formula I has the formula
 17. A synergistic combination as claimed in claim 16 in which the compound of formula VII is:
 18. A synergistic combination as claimed in claim 1 consisting essentially of a mixture containing methylamino di(methylene phosphonic acid) and from 30 to 85% by weight of sodium nitrite.
 19. A synergistic combination as claimed in claim 1 consisting essentially of a mixture containing hydrazine tetra(methylene phosphonic acid) and from 22% to 30% by weight of sodium nitrite.
 20. A synergistic combination as claimed in claim 1 consisting essentially of a mixture cOntaining Alpha -anilino ortho hydroxy benzyl phosphonic acid and from 30% to 85% of sodium nitrite.
 21. A synergistic combination as claimed in claim 1 consisting essentially of a mixture containing ethylene diamine tetra (methylene phosphonic acid) and from 25% to 85% sodium nitrite.
 22. A synergistic combination as claimed in claim 17 consisting essentially of a mixture containing a compound of formula VIII and from 20% to 80% sodium nitrite. 